early 14c., "declare to be untrue or untenable," from Old French denoiir "deny, repudiate, withhold," from Latin denegare "to deny, reject, refuse" (source of Italian dinegarre, Spanish denegar), from de "away" (see de-) + negare "refuse, say 'no,' " from Old Latin nec "not," from Italic base *nek- "not," from PIE root *ne- "not."
From late 14c. as "refuse, refuse to grant or give," also "refuse to acknowledge, disavow, disown." Sense of "refuse access to" is from 1660s. Related: Denied; denying.
I may not understand what you say, but I'll defend to your death my right to deny it. [Albert Alligator, "Pogo," Sept. 26, 1951]